The present invention is directed to a device for determining the proper revolutions per minute (RPM) for cutting or milling a particular material.
When conducting machining operations on a particular workpiece, it is important to determine the correct RPM of the cutting tool which is based upon surface foot rating of the material constituting the workpiece. It is well known that the surface speed of the material, calculated in a surface foot rating, has a definite relationship with respect to the diameter of the cutting tool and the RPM of the cutting machine. Various tables have been developed showing this relationship. Therefore, utilizing these tables for specific surface speed would convey information relating to the proper diameter of the cutting tool and the speed of rotation of the cutting tool. Once this information was determined, the operator of the machine would select the diameter of the cutting tool, insert it into the machine, then would manually set the proper RPM of the spindle to which the cutting tool is attached.
Furthermore, if the operator wishes to perform several successive operations on the workpiece utilizing tools of different diameters, the proper RPM must be calculated or read from the table for each of the tools. For example, if the operator wishes to drill a one-half inch diameter hole in a material, he must calculate the RPM based on the diameter of the cutter and the surface foot rating of the material to be machined. Subsequently, if the operator wishes to drill a one-inch diameter hole in the same material, the operator must change the RPM of the spindle, based upon the calculation of the surface feet per minute of the material to be machined. Not only would these calculations have to be performed, they must be retained by the operator so that he would not have to calculate the RPM of the spindle if the same operations were to be performed on more than one workpiece.
Most manual knee-type milling machines utilized at the present time employ two methods of changing the spindle speed (RPM). The first method would include a mechanical variable-speed drive in which the operator changes the RPM by a crank handle which, in turn, changes the belt position on a bellows-type pulley arrangement. A mechanical dial associated with the milling machine would display the particular spindle speed. Unfortunately, although the particular speed of the spindle would be displayed, the actual spindle speed could vary substantially, based upon the mechanics of the design.
The second method which is presently utilized for changing spindle speeds would employ a step pulley. Although this step pulley would provide a more accurate spindle speed, it is limited to a selection of only several discrete RPMs.
A number of prior art patents have issued relating in general to the problem of determining the proper operating parameters for a particular endeavor. For example, U.S. Pat. No. 2,942,781 issued to Abatemarco relates to calculating and analyzing devices which are adapted to utilize data obtained from standard tables to determine optimum operating conditions. In a machine tool such as a lathe or milling machine, the fundamental relationship between various parameters such as time, length of cut, feed rate, rotary speed of the work or cutter, the number of teeth on the cutter, the chip load per tooth per revolution as well as the relative surface speed of the cutter and work and the diameter of the cutter or work are known. Therefore, if one or more of these parameters are inputted into the standard data analyzer shown in the Abatemarco patent, parameters dependent upon those which were introduced into the analyzer can then be determined. However, the calculator device of the Abatemarco patent has no control over the actual speed of a milling machine spindle.
Furthermore, as stated in column 7 of the Abatemarco patent at line 47, xe2x80x9cIt is assumed, of course, that the person operating the analyzer will have access to the necessary books, tables or charts containing the information which is pertinent to his problem. He will need to know, for example, the recommended surface speeds and chip loads for specified machining operations performed upon various given materials under the different conditions that are likely to be encountered in practice. The available speeds and feed rates of standard machine tools usually are known or can easily be obtained. The local shop practice, insofar as it affects the parameters of the problem, also must be taken into consideration.xe2x80x9d The fact that look-up tables are required as recited at columns 8 and 9 of the Abatemarco patent.
Similarly, U.S. Pat. No. 4,707,793 issued to Anderson describes a method of determining the feed rate and cutting speed for cutting metal by inputting various parameters into a calculator. Any calculations produced by the Anderson calculator would have to be manually inputted into a particular machine.
As indicated by the Anderson patent, these initial parameters are obtained from reference manuals which state known recommended starting point machining recommendations. These recommended surface foot ratings and cutter tip chip loads based on the material being machined, and type of cutting tool being applied to machine the work piece, are monitored manually in order to determine maximum efficiency of the machining process. The basis of this optimum machining is related to the known horsepower of the machine spindle, the fixturing of the work piece, rigidity of setup, mechanical configuration of the attached cutting tool, and finish requirements of the completed work piece.
U.S. Pat. No. 5,136,903 issued to Hibi describes a method of automatically determining a machining range such as the best chucking point on a workpiece machine utilizing a CNC program. However, as was true with respect to the Anderson and Abatemarco patents, this patent does not deal with the peripheral speed of a rotating cutter used in a milling machine.
U.S. Pat. No. 4,698,773 issued to Jeppsson is directed to a system for adaptively limiting the feed rate of a milling machine""s positioning axes during a milling operation. Strain gauges are employed on a numerically controlled milling machine which senses the load on the cutting tool. A signal is then sent to a device for automatically overriding the program feed rate to maintain a peak side load on the cutting tool. As was true with respect to the above-noted patents, the Jeppsson device does not control the speed of the spindle but only overrides the feed rate.
The disadvantages of the prior art are addressed by a first embodiment of the present invention which would remove all guess work and manual calculations from machining on the type of milling machines in which various workpieces are treated. (Although this invention will be applied to other operations, i.e. lathes, etc., for the present time, the milling machine is addressed here.) The operator would be only required to know the surface foot rating of the material to be machined. This parameter is entered into the programmable speed controller of the present invention which would display the correct spindle speed (RPM) for the diameter of the cutter the operator selects. For example, when the operator rotates an analog variable-speed dial, the displayed RPM increases or decreases, based upon the diameter of the cutting tool which is simultaneously displayed. Therefore, according to the present invention, once the surface foot rating is entered into the device and a cutting tool having a particular diameter is selected, the speed of the spindle would also be displayed.
The speed of the spindle for a selected cutting tool is then converted into a direct current (DC) value which is transmitted to an alternating current (AC) inverter. The DC value of the particular spindle speed is then converted into an AC motor which rotates the spindle. This spindle speed is directly related to the AC output of the AC inverter which in turn operates the AC motor at the same value to produce a very accurate spindle speed. This embodiment is described in U.S. Pat. No. 5,914,881, granted to the applicant of the present invention.
A second embodiment of the present invention would include an extended read-only memory (ROM), a programmable read-only memory (PROM), and erasable programmable read-only memory (EPROM) or similar memory device which would include the surface foot rating for a number of materials. This embodiment could take the form of a portable, compact computing device as a stand alone device, or in combination with the above-described first embodiment. This second embodiment would calculate the required RPM for a machine tool based upon the material and the type of tool cutter being used. This device allows the user to scroll through a list of pre-programmed materials and pre-programmed machine tools. Upon selecting the tool diameter, this second embodiment will display to the operator via a display device such as a liquid crystal display (LCD) the required RPM to operate the machine. At this point, the operator could manually set the machine to the proper RPM or, alternatively, this second embodiment would be in direct communication with the particular machine which would automatically set the tool to the proper RPM.
Further features, advantages, and benefits of the present invention will be found in and perceived from the ensuing detailed description of the preferred embodiments of the present invention. The drawing which accompanies the disclosure illustrates the preferred embodiments in the best mode contemplated at this time for carrying out the invention.